Oscillator circuits



Sept. 27, 1949. 5, CQHEN 2,483,183

OSCILLATOR CIRCUITS Filed June 25, 1947 S'Shets-Sheet l Sept. 27, 1949.

OSCILLATOR CIRCUITS Filed June 25, 1947 3 Sheets-Sheet 2 47 a fizz/ 2%;]

Z5 Eff/ y 55 J. s. COHEN 2,483,183 7 Sept. 27, 1949. J. 5. COHEN Q 2,483,183

OSCILLATOR CIRCUITS Filed Juno 1947' '3 Sheets-Sheet 3 Patented Sept. 27, 1949 UNITED STAT PATENT?" 2,483,183

O C LL T CIRCU T Jeffrey S. Cohen, Los Angel'es, Galiit, assignonto Avco Manufacturing Corporation, Cincinnati;

Ohio, a. corp ra ion oflBelaware.

Ap l at o J 7 ia N0.- 1 6, 9

9 Claims.

1 The present invention relates to alternating current electrical generators, particularly to elec: tromagnetic wave signal generators, and, it has a wide range of prospective application, specifie cally to a tunable local oscillator particularly oscillator operating at 6365 kilocycles may be employed, in conjunction'with a mixer, to develop intermediate frequency signals having a frequency of, say, 5825 kilocycles. Since thestandard broadcast band extends to 1600 kilocycles, the local oscillator should be tunable between 6385 and 7425 kilocycles, for broadcast reception.

When receiving 9.45 megacycle short-wave car-- rier signals an oscillator operating at 1-5'.2'75 megacycles may be used, for example. For a shortwave band extending to 11.9 megacycles, the oscillator should be tunable between 15.2 75 and 17.725 megacycles, when the same intermediate frequency of 5,825 megacycles is required.

When receiving 88.1 megacycle frequency modulated signals, for example, a local oscillator operating at 98.8 megacycles may be used, assuming an intermediate frequency of 10.7 megacycles. This oscillator should be tunable between 98.8 megacycles and 118.6 megacycles, for a frequency modulation band extending to 10719 megacycles.

The above mentioned parameters are illustrative but they show that in a three-band receiver having such parameters three oscillator frequency range requirements must be met:

First, a variable source of local oscillations within the frequency range 6365 to 7425 kilocycles is required, for broadcast (BC) ODB IiQI J Second, a variable source of local Oscillations within the frequency range 15.275 and 17.125 megacycles is required for short wave (SW) operation;

Third, a variablev source of local oscillations within the frequency range 98.8 to 118.6 megacycles is required, for frequency modulation (FM) signal reception.

In effect, a three-band receiver requires three o a c ation ene a in s em It. wi l be seen that, in substance, three diilerent oscillator circuits are required. It is desirable to employ a common electronic oscillator tube for. these. three systems. The objective of quality radio. receiver-manufacturers to furnish the. most radio with the least-number of componentsand tubes, minimizing costs to the consumer andv service troubles. When employing a common tube, the various f-requency determining. circuit elements, such as inductances and capacitances, have heretofore had to be whollymechanically, switched in and.- out of circuit association with. that tube in order to obtain the required three types of operation. It is also desirable to em: ploy the same tuning condenser during the, three types oioperation andiully to utilize com: mon circuit elements wherever possible. These and other requirements, however, have. created dilemma which the radio industry has heretofore tried in vain to solve, in that mechanicalswitch contacts have heretofore been introducedin the PM or U. H. F. frequency-determining and oscillator circuits when the desired objectives were sought, It is urgent from a standpoint of stability and reliability of performance that, When the oscillator is switched i nto or U. H. F. operation, no. current-carrying or hot switch contacts, be included in its circuit. 7

oscillator for FM or U, H. F. receivers is a xtrem ly sen it e and r ca de e nd ex a e us ta ers. pa icularl w t h cont ts impair its. ability I s i tha on e the receiver tu ed or he rec ptie e si n f m. a s ren. ation the en r ed frequ ncy of the es il e ie bee e r y cons an asro s e Va ious. fac rs. tend; to aus tha frequency to chan a ng ari tion, o n uc ance and capacitance wi temee a re var lio in the intereiectrodecanaeity or, the e eilla qr t be as a result of change tfimperature or tube re.- placement, variations the load reactance coupled into the oscillator, variations. in supply voltage, and soon. Bycareiul design the efiects of many of-these undesired factors may be mini: mized. However, frequency stability is so ad,- versely and drastically affected .by the presence of switch contacts in the PM or U. H. F. Oscillator circuits that it has heretofore been considered necessary to employa separate and distinct oscillator tube for proper-EM operation, rather than to switch circuit components into and out of electrical association with a common tube. Such switch contacts cause very serious varia: tions in the resistance, oapaeitanee and inductance of the irequency determining or tank circuit of the FM or U. H. F. oscillator and upset its stability.

It is, accordingly, the principal object of the invention to provide a simple means for adjusting a common oscillator tube from low (SW or BC) to high (U. H. F. or FM) frequency operation without including current-carrying switch contacts in its frequency-determining circuits during FM or U. H. F. operation.

Another basic object of the invention is to provide an oscillator circuit, suitable for the abovementioned two or more types of operation, and having a single tube or tube section and a minimum number of parts or circuit elements.

A further important object of the invention is to provide such an oscillator circuit, so arranged that the same tuning capacitor may be employed during the two or more types of reception.

Other and further objects and capabilities of the invention will be apparent from the following description of the accompanying drawings in which there are illustrated two preferred embodiments of multi-purpose oscillators in accordance with the invention.

In the drawings, Fig. 1 is a diagram showing the active circuit components of an illustrative oscillator circuit in accordance with the invention during the U. H. F. or FM operation; Fig. 2 is an electrical equivalent diagram of the Fig. 1 circuit; Fig. shows the circuit components during BC operation; Figs. 4, 5, and 6 are simplined electrical equivalent diagrams of the Fig. 3 circuit; Fig. 7 shows the circuit components during SW operation; Fig. 8 is a simplified electrical equivalent diagram of the Fig. 7 circuit; Fig. 9 is a diagram showing all the circuits of a commercially successful form of three-purpose oscillator in accordance with the invention; Fig. 10 is a sectional view showing the U. H. F. tank circuit inductance and certain associated capacitors included in either embodiment; and Fig. 11 is a simplified form of dual-purpose oscillator circuit in accordance with the invention.

Referring now first to Fig. 10, there is provided a U. H. F. tank-circuit inductance I2, formed as a single metallic loop. It may be sil verplated to minimize R. F. losses. The terminals of this loop are conductively secured to the separate stator portions l3 and 14 of a splitstator tuning capacitor or variable condenser 15. The rotor portions 16 and ll of this condenser are mounted on a common grounded shaft 18. Bridged across the loop is a capacitor 2i, having its plates 59 and 2B individually conductively connected to the legs of the loop. The inductance of inductance l2 and the parallel capacitance including that of units 2| and 29 are such that at the U. H. F. range, such as 118.6 megacycles, for example, a high parallel-resonant circuit impedance exists between points X and Y, that is, between the loop terminals and between stator portions l3 and M. It will be seen that inductance l2 and the portions [a and "lb of condenser [E are then equivalent to an inductance in parallel with a series combination of two capacitors I50. and Nib, i. e. to the tank circuit of a Colpitts oscillator. This tank circuit is tunable by rotation of shaft [8. Rotor 18 of condenser 15 is grounded.

On the other hand, when the generated oscillations are on the order of 6365 to 7425 kilocycles, say, for BC operation, or 15.275 to 17.725 megacycles, say, for SW operation, the impedance of the theretofore anti-resonant circuit including inductance [2 decreases to such an extent that stator portions l3 and M are effectively connected to each other, inductance [2 then being a virtual short circuit and capacitors 2i and 29 being shorted out. The circuit elements i2, 2!, 29 and 15a and IE1) then become equivalent to a variable condenser, portions hire, 5% being in parallel, with the common rotor terminal P grounded and the other terminal Q common to both stator portions. Without any mechanical switching Whatever, in the Fig. 1G or Fig. 1 tank circuit elements, then, that which is effectively a capacitance during BC and SW operation, becomes a Colpitts tank circuit during the U. H. F. or FM operation. Conversely, that which is a Colpitts tank circuit during U. H. F. or FM operation, effectively becomes a capacitance during BC or SW operation. With this key in mind, the detailed description of the circuits proceeds.

The capabilities of a parallel combination of inductance and capacitance (i. e., to act like a Very high impedance or resonant circuit at U. H. F. frequencies, but like a short circuit at lower BC or SW frequencies) are here utilized. Such a combination is a frequency-responsive non-mechanical switch.

Referring now specifically to Fig. 1, there is shown a Colpitts capacitive feedback oscillator particularly suitable for U. H. F. operation. It comprises an oscillator tube 23, employing a shunt-fed plate-supply voltage, the anode being connected to a suitable source of space current through a resistor 24 or the like. The grid resistor 25 is effectively connected between grid and ground, since it is connected to a center tap on loop i2, through the upper portion 26 of an inductance 21. An iron core '28 is provided ior the inductance I2. Capacitor 2! projects through and is snugly mounted in openings in the legs of loop 12. In addition to capacitor 2 l the plates of which are individually soldered to the terminals of loop [2, there may be provided a trimmer capacitor 29, in parallel with capacitor 21. The tank circuit comprises inductance l2,

4,5 the split-stator condenser l5 (equivalent to two ganged variable condensers Ma and its!) in series), with their junction grounded, and the parallel combination of capacitors 2i and E9. Tuning is accomplished by turning rotor shaft I8 of ganged condensers i501. and i517. The tank circuit terminals are connected to the plate and grid of tube 23, the plate connection bein made through series blocking capacitors 3i and 32. The purpose of capacitor 34 is hereinafter can plained in more detail. Capacitors 53 and 5 on converter tube.

are provided for oscillator coupling purposes. For U. H. F. it was found preferable to couple from the grid side of the oscillator, and capacitor 53 serves to couple the oscillator voltage to the Capacitor 54 is open circuit on U. H. F.

As clearly shown in Fig. 9, wipers 33 and 3d are in position F during U. H. F. or FM operation, so that all circuit components (other than those shown in Fig. 1) are entirely disconnected from the oscillator circuit. No current-carrying switch contacts whatever are then in the U. H. F. oscillator circuit. The point of interconnection Q of inductance I2 and inductance 27 is at ground potential for U. H. F. signals, and that inductance 21 in no way affects the U. H. F. oscillations. Further, since wiper 33 (Fig. 9) is in position F, inductance 27 is open circuited. It will be seen, then, from the electrical equivalent :5 circuit shown in Fig. 2 that the oscillator functime as a conventional. Colpitts. circuit during U. H; F. operation, loop. l2: then being the tank circuit inductor.

BC operation is next considered.

Wipers 33 and 33 are ganged, as by any suitable expedient indicated, by. the dashed line 31. (Fig. 9).. As stated above, during BC or SW oper-' ation, inductor |2. and capacitors 23-, I5 and. 2| are equivalent to a variable condenser (150., I51) in parallel), having one common terminal P grounded. The other common terminal Q is con-- nected to the center tap of. inductance 21. All parts of the cross-hatched area in Fig. 3.are then at. the same R. F. potential as indicated in the description of Fig. 10. Both parallel parts of condenser l5 now function as one of the tank circuit capacitances. It will be recalled that. a Colpitts oscillator tank circuit requires. an inductance and two series capacitors, forming a resonant circuit. Inductance. 2-1 is the required inductance. Capacitors 36--35 comprise the other required tank circuit capacitance. Inductance 21 functions as the tank circuit inductance. To switch in said other tank circuit capacitance, wiper 33 is placed as shown inposition B, connecting the ungrounded terminal of the parallel combination of capacitors 35, 36 to the remaining terminal of inductance 21. The junction ofinductance 21 and condenser I5 is already connected to the grid. But inductance 21 must be placed across the grid and plate of tube 23. To couple the junction of inductance 21 and condensers 35, 35 to the plate, wiper 34 is placed as shown in position B, whereby that junction is connected to the junction of capacitors 3|, 32. Capacitor 32 now functions as a blocking. condenser and capacitor 3| is in parallel with the. tank circuit inductance 21. Oscillator voltage is now delivered to the convertor tube, through coupling condenser 54, condenser 53 being open circuit. In addition, oscillator plate voltage is reduced by removing the short circuit from re,- sistor 24A and thus placing resistors 24 and 24A in series.

Figs. 3, 4, 5, and 6 show progressive circuit equivalents for the BC oscillator circuits. In Fig. 4 the wipers 33, 34 are deleted, grid resistor 25 is transposed and capacitors 35, 36 are combined. In Fig. 5, capacitor 3| is placed in an equivalent electrical location and capacitors Hi and H are combined. In Fig. 6, the parts are rearranged, in order to show graphically that the net result of the positioning of wipers 33 and 34 is to produce a simple Colpitts oscillator circuit, with inductance 21' and capacitors 3|, 35, 3B and both parallel parts of condenser l5 as its tank circuit.

For SW operation wipers 33 and 34 are placed in theS position (Fig. 9). An inspection of Figs. 7 and 8 will show that a simple Colpitts oscillator is again produced with portion 26 only of inductance 21, and capacitors 3|, 38 and both parallel parts of condenser l5 as its tank circuit. One terminal of inductance 26 is already efiectively connected to the ungrounded terminal Q of condenser l5. Wiper 33 connects the other terminal of inductance 25 to the ungrounded terminal of a capacitor 38 and their junction point is connected by wiper 3 to the junction of condensers 3|, 32 in order to place the tank circuit across the grid and plate of tube 23. The essential difierence between the SW and BC circuits is that during SW operation inductance portion 26 and condenser 38 are substituted for inductance 21 and condensers 35, 36 respectively.

The ungrounded junction of capacitors 36 35 6: is. connected to terminals; 4.0? and: 46., The ungrounded terminal; of. capacitor 38 is connecte to. terminals, M and 41;. The terminal of inductance 2-;1; remote from point Q on; coil I2 is connected. toterminal. 48;. The tap terminating inductance portion. 26 is connected to terminal 39. The junctionof; condensers 3|, 32 is connected to. spaced terminals. 43, 44. During BC operation wiper 34. connects terminals 40 and 43 and? wiper .33 connects terminals 4.6 and 48. During SW operation wiper 34 connects terminals 4|, and 44. and: wiper 33. connects terminals 4.1 and 49 During FM operation condensers 35, 35, 3.3, wipers 331and 34: and terminals 46, 41-, 40 and 41. have no operative effects on the oscillator.

Explanation of the construction and operation of a Colpitts oscillator per se is not herein required', since suchoscillator is well known to the art and described forexample, in: Applied Elece t-ronics, M. I. T. Staff, page 617 et seq, Wiley, New York, 1943;- Radio Engineering, Terman, page 349 et seq., McGraw-I-Iill, New York, 1937; Theory. and Applications of Electron Tubes, Reich, page 391 et seq., McGraw-Hill, New York, 1944.

In one successful commercial embodiment of the invention operating on the above mentioned frequencies the following illustrative circuit parameters were found to be suitable:

Capacitor 32 91 mmf., 300 volts, ceramic Capacitor 3 39 mmf., 300 volts, ceramic Resistor 25 22,000 ohms, watt Condenser |5 Two variable sections, each 57 mmf. max. Capacitor 2-| 50 mmf., 500 volts, ceramic Resistor 24 10,000 ohms1 watt Resistor 24A 20,000 ohms, 1 watt Capacitor 29 3-30. mmf., air trimmer Capacitor 38 330 mmf., air trimmer Capacitor 3.6 3.-30 mmf., air trimmer Capacitor 35 10 mmf., 300 volts, ceramic Tube 23 Section of Type 7F8 Triode Inductance |2 Strap, wide, 1" dia., silver plated Inductance 2'l. Coil 39% turns on .375 O. D. inch form, #30 B & S ga. enameled copper wire, Wound 52 turns per inch.

During BC and SW operation, condenser i5 is used as. a single-ended capacity. The condenser |5is used for tuning the oscillator during all three types of operation. Only one oscillator tube is used. Noswitch contacts are present in the U. H. F. frequency-determining circuit. While the Fig. 9 circuit is simple and inexpensive, it overcomes the disadvantages and limitations of;,prior art. circuits.

Referring now to Fig. 11, wherein a very simple form of the invention, is shown, it will be seen that. elements 23, |2, 29, 2|, l5fa, I51) and 32 comprise the U. H. F; oscillator. A parallel combination of inductance 2'1' and capacitance 3| is interposed between any ungrounded part of the U. H. F; resonant circuit I2, 23, 2|, |5a, I51) and either the anode or grid of tube 23. In the illustrated form shown, the combination is interposed between the anode (which. is blocked for D. C. by capacitor 32) and the junction of coil l2 and capacitor [5b. The parameters of elements 21 and 3| may be made such that the U. H. F. operation is not appreciably affected by their presence. When BC operation is desired, for example, capacitor 35, 36 is connected betweenground and the junction of inductance. 421 and capacitor 32.

(Ifthe al oov'ementioned 21, 3| combination is interposed in the grid lead, then the capacitor 35, 36 is connected between ground and grid.) During BC operation the tank circuit comprises inductance 21, capacitors 3|, l5a, I51) and 36, 35. In one aspect, then, the invention comprises a U. H. F. Colpitts oscillator having an electron tube, a U. H. F. L-C tank circuit, a parallel combination of an inductance and capacitor interposed between either the grid or plate of the tube and said tank circuit, a capacitor, and means for shunting said capacitor across the corresponding one of either the grid or plate circuits of the tube, the parameters of said inductance and capacitance being such that, together with the variable condenser elements in said L-C tank circuit, they function as a lower-frequency Colpitts oscillator circuit.

While there has been described what is at present considered the preferred embodiment of the present invention, it will now be obvious to those skilled in the art-who have the disclosure before themthat various modifications and substitutions of equivalents can be made without departing from the true scope of the invention. It is, accordingly, intended in the appended claims to cover all such changes and modifications as fall within the true scope of the invention. For example, it is obvious to those skilled in the art that the tube leads could be reversed at points 56, 5| (Fig. 9). That is, the terminal of plate condenser 32 remote from the anode could be connected to the junction of coil 12 and condenser l5a, and the terminal of condenser 3| remote from loop l2, together with contacts 44, 43, could be connected to the grid of the tube 23. In that case, resistor 25 would be connected directly to the grid. Further, while the Cclpitts type of oscillator was chosen for purposes of illustration, the invention is not limited to that particular type. Also, it will be obvious that separate coils could be used for SW and BC in place of the tapped coil 21.

Having thus described my invention, I claim:

1. In combination, an oscillator tube, a parallel circuit between its anode and control electrode comprising an inductance adapted to function as a short circuit at standard AM broadcast frequencies, and including a pair of series-connected capacitors bridged across said inductance with their junction effectively connected to the cathode whereby said inductance and said capacitors are adapted to function as an oscillator tank circuit at ultra-high frequencies but as a pair of parallel capacitors at AM broadcast frequencies, another inductance having a terminal connected to the first-named inductance, a pair of capacitors interposed between the anode of said tube and said parallel circuit, a fifth capacitor having a terminal effectively connected to the cathode of said tube, switching means for simultaneously connecting the remaining terminal of said other inductance to the remaining terminal of said fifth capacitor and to the junction of the lastmentioned pair of capacitors, said other inductance and said fifth capacitor and said parallel capacitors having such parameters that they are adapted to function as an oscillator tank circuit at frequencies within the range whereat the first-mentioned inductance functions as a short circuit.

2. A dual purpose oscillator for upper band and AM broadcast operation, having no mechanical switch contacts among its upper band frequencydetermining elements comprising: An electronic tube having a D. C. blocked anode, a series combination of an inductance and a blocking capacitor bridged across the anode and control electrode of said tube, a pair of series-connected capacitors bridged across said inductance, said series-connected capacitors having a junction effectively connected to the cathode of said tube, another inductance having a terminal connected to the first-named inductance, another capacitor having a terminal effectively connected to said cathode, and circuit-making-or-breaking means for connecting together the remaining terminals of said other capacitor and said other inductance and for connecting said other inductance across said anode and control electrode, said other inductance and said series-connected capacitors and said other capacitor having such parameters that they function as an oscillator tank circuit at AM broadcast frequencies while said circuitmaking-or-breaking means is closed, said tuned inductance having such parameters that it acts as a short-circuit and connects said series connected capacitors in parallel at AM broadcast frequencies, said circuit making or breaking means. being operative to disable said other capacitor when high frequency operation is desired, the first-named inductance and said series-connected capacitors having such parameters that they function as an oscillator tank circuit for upper band reception.

3. A dual purpose oscillator in accordance with claim 1 wherein said series-connected capacitors are variable in synchronism to tune the oscillator during all above-mentioned types of operation.

4. A two-band oscillator comprising an electronic tube, an inductance between plate and grid of said tube, a pair of capacitors one of which is in circuit between plate and cathode of said tube and the other of which is in circuit between grid and cathode of said tube, said inductance and pair of capacitors being comprised in a resonant high-frequency tank circuit, a second inductance and third capacitor and means interposing them between said tank circuit and one of the cold electrodes of said tube, a fourth capacitor, and means for efiectively connecting said fourth capacitor between said cold electrode and the cathode during lower-frequency operation, the said pair of capacitors being effectively connected between the other cold electrode of said tube and its cathode by said first inductance during lower-frequency operation and said firstinductance becoming a short-circuit during such operation, whereby said second inductance and said pair of capacitors and said fourth capacitor function as a tank circuit at said lower frequencies.

5. In an oscillator for two Widely spaced bands, the combination comprising a vacuum tube having at least a cathode and cold anode and control electrodes, a high frequency oscillationcircuit for the upper band comprising a parallel combination of a first inductance and a series-connected pair of capacitors across said inductance, said inductance being a short circuit for the lowfrequency band, said capacitors each having one terminal effectively connected to said cathode, a second or low-frequency inductance having one terminal conductively connected to said first inductance, a third capacitor in series with said first inductance, means for coupling said second inductance and said third capacitor in parallel between the remaining terminals of said pair of capacitors and a cold electrode, a fourth capacitor, switching means for effectively connecting said fourth capacitor between said cathode and said one of said cold electrodes, said second inductance and fourth capacitor and said pair of capacitors comprising a low-frequency oscillation circuit adapted to be selected by the operation of said switching means, and means including a fifth or blocking capacitor for R. F. coupling said first inductance across said cold electrodes.

6. In an oscillator for two widely spaced bands, the combination comprising a vacuum tube having at least a cathode and anode and control electrodes, a high frequency oscillation circuit for the upper band comprising a parallel combination of a first inductance and a series-connected pair of tuning capacitors across said inductance, said inductance being a short circuit for the lowfrequency band, said capacitors each having one terminal effectively connected to said cathode, a second or low-frequency inductance having one terminal conductively connected to said first inductance, a third capacitor, means for coupling said second inductance and said third capacitor in parallel between the remaining terminals of said pair of capacitors and said control electrode,

a fourth capacitor, switching means for effectively connecting said fourth capacitor between said cathode and said control electrode, said second inductance and fourth capacitor and said pair of tuning capacitors comprising a low-frequency oscillation circuit adapted to be selected by the operation of said switching means, and to be tuned by said pair of tuning capacitors, and means including a fifth or blocking capacitor for R. F. coupling said first inductance between said anode and control electrodes.

'7. In an oscillator for an FM and short-wave two band receiver two widely spaced bands, the combination comprising a vacuum tube having at least a cathode and cold anode and control electrodes, a high frequency oscillation circuit for the upper band comprising a parallel combination of a first inductance and a series-connected pair of tuning capacitors across said first inductance, said inductance being a short circuit for the low-frequency band, and said capacitors each having one terminal effectively connected to said cathode, said inductance having such a value that during the lower-frequency reception it effectively connects said tuning capacitors in parallel between one of said cold electrodes and, said cathode, means including a switch added to such tube and a second inductance and pair of capacitors for completing an oscillating circuit for the lower-frequency band, and means for permanently R. F. coupling said first inductance between said anode and control electrodes, said second inductance having one terminal permanently conductively connected to said first inductance.

8. In an oscillator for two bands, a combination which acts as an oscillator for a high frequency band and a low frequency band comprising a vacuum tube having at least a cathode and cold anode and control electrodes, a high frequency oscillation circuit for the upper band comprising a parallel combination of a first inductance and a series-connected pair of gang tuning capacitors across said inductance, said capacitors each having one terminal effectively connected to said cathode, said inductance being a short circuit for the low-frequency band, whereby said capacitors are effectively connected, for lower band operation, between said cathode and one of said cold electrodes to function effectively as a tuning capacitor for the low-frequency band, means for R. F. coupling said first inductance between said anode and control electrodes, and means including a switch and a second inductance having a terminal permanently conductively connected to the first-mentioned inductance for cooperating with said tuning capacitor to function as a low frequency tank circuit.

9. A three-terminal tank circuit suitable for use in a two-band oscillator comprising a high frequency oscillation circuit for the upper band consisting of three terminals and a parallel combination of a first inductance having a high impedance at the high frequency band and a series connected pair of gang tuning capacitors across said inductance, said pair of capacitors and first inductance acting as a high frequency tank circuit, said pair of capacitors having a junction which forms one terminal, a third capacitor in series with said first inductance, one end of said first inductance and one terminal of said third capacitor forming the second and third terminals, respectively, said inductance being a short circuit for the low frequency band, whereby said pair of capacitors are effectively connected in parallel for low band operation between said first terminal and either of the other two terminals to function effectively as a tuning capacitor for the low frequency band, and means including a switch and a second inductance bridged across said third capacitor and having at least one terminal permanently conductively connected to the firstmentioned inductance for cooperating with said tuning capacitor to function as a low frequency tank circuit, the last-mentioned means also including a fourth capacitor adapted to be connected by said switch between said first terminal and the other one of said two terminals to complete a low frequency tank circuit including at least said second inductance, said pair of tuning capacitors and said fourth capacitor.

JEFFREY S. COHEN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,559,802 Stevenson Nov. 3, 1925 2,192,978 MacNabb Mar. 12, 1940 2,254,739 Grayson Sept. 2, 1941 2,292,254 Van Beuren Aug. 4, 1942' 2,434,299 Van Loon Jan. 13, 1948 

